Illumos 6319 - assertion failed in zio_ddt_write: bp->blk_birth == txg

Illumos 6319 - assertion failed in zio_ddt_write: bp->blk_birth == txg

6319 assertion failed in zio_ddt_write: bp->blk_birth == txg
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/6319
  https://github.com/illumos/illumos-gate/commit/b39b744

Porting notes:
- Re-enabled ztest for CentOS test slaves.

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3449